Surface light scattering from cationic surfactant films

Surface light scattering (SLS) is shown to be a powerful and versatile meth od for investigating various properties of surfactant films at fluid interf aces. Interfacial capillary and dilational waves in films of cationic surfa ctants, with different hydrophobic chain structures, have been studied by S LS over a frequency range 10(4)-10(5) s(-1). At the air-water interface, wi th either C(16)TAB [C16H33N(CH3)(3)Br-+(-)] or a di-C16 gemini analogue {C4 H8-alpha, omega-[(CH3)(2)N+C16H33Br-](2)}, analysis of the correlation func tions in terms of the dispersion equation gave negative values for the dila tional viscosity epsilon'. This behaviour implies the dilational waves are destabilised, and is consistent with theory accounting for an adsorption ba rrier [Hennenberg et al., J. Colloid Interface Sci. 150 (1992) 7]. A switch to positive epsilon' occurred when dodecane was added into a C(16)TAB film , and this indicates a higher barrier owing to the increased film density. SLS has also been used with small-angle neutron scattering (SANS) to determ ine bending rigidities of di-chain cationics at oil-water interfaces in Win sor II microemulsions. This rigidity was the sum 2K+(K) over bar, where K i s the normal bending modulus and (K) over bar the topology-dependent Gaussi an modulus. The values of 2K+(K) over bar were between 1.0 and 2.5 k(B)T an d they increased with surfactant chain length t and added NaBr electrolyte. For salt-free systems a scaling relationship (2K+(K) over bar)similar to t (2.3) was observed which agrees reasonably well with theory [Szleifer et al ., J. Chem. Phys. 92 (1990) 6800]. Furthermore, for NaBr concentrations bet ween 0.00625 and 0.10 mol dm(-3) the dependence of 2K+(K) over bar on elect rolyte concentration was as predicted by Lekkerkerker [Physica A 159 (1989) 319]. (C) 1998 Elsevier Science B.V. All rights reserved.